1. The Field of the Invention
The invention relates to a process for the production of a strongly adhering and wipe-proof, porous SiO2-containing antireflection layer on a borosilicate glass body, i.e. a process for the coating of a borosilicate glass body with a strongly adhering and wipe-proof porous SiO2-containing anti-reflection layer.
2. Description of the Related Art
It is known that porous SiO2 layers adhere poorly to borosilicate glass bodies. This is true in particular of layers which have been produced with the use of a SiO2 sol.
For improving the adhesion, it has already been proposed to add to the sol tetraethyl orthosilicate whose decomposition products bond the SiO2 particles to one another and to the substrate (U.S. Pat. No. 2,601,123). However, these solutions are complicated to prepare and are relatively expensive.
EP 0 897 898 B1 discloses a process using a purely aqueous coating solution which contains SiO2 sols and surfactants. In this process, it is absolutely essential that the substrate to be coated be pretreated with acetone, ethanol and water, a strongly alkaline cleaning solution (1N NaOH) or commercially available cleaning baths, it being necessary if appropriate to reinforce the cleaning effect by the use of ultrasound. Particularly the cleaning steps (see Example 1) make this process expensive.
EP 1 342 702 A1 or U.S. Pat. No. 6,998,177 B2 discloses a process which is advantageous in terms of process engineering and in which an alcohol-containing, nitric acid-stabilized suspension of a SiO2 sol is used. The known poor adhesive strength and wipe resistance of the SiO2 particle layer formed are avoided by adding H3PO4 to the coating liquid. The adhesive strength and wipe resistance of the antireflection layer thus produced is very good, but it has been found that such layers age in the course of a few weeks, which manifests itself in that the originally deep blue antireflection layers fade and become cloudy so that the desired high transmittance for incident light and the wipe resistance decline. This fading can be reversed by washing with water. It can be avoided if the glass bodies provided with the antireflection layers are washed with water after their production in the factory itself, with the result that the aging problem is eliminated in practice. However, this washing step means an undesired additional expense.